The present invention relates generally to wireless communications systems and, more particularly, to systems and methods that optimally assign pseudo-noise (PN) offsets to base stations in a Code Division Multiple Access (CDMA) cellular network to maximize co-offset and adjacent offset protection and to minimize interference in the network.
A CDMA cellular network is a digital spread spectrum communications system. The CDMA network includes several base stations that provide digital service to wireless units located in different geographical regions. Communication between a wireless unit and a base station in a CDMA network, based on the IS-95A standard, occurs on reverse and forward CDMA channels. The reverse CDMA channel carries traffic and signaling information from a wireless unit to a base station. The forward CDMA channel carries pilot, sync, and paging signals, in addition to traffic signals, from a base station to a wireless unit.
The reverse CDMA channel includes access channels and reverse traffic channels. The wireless unit uses the access channels to initiate communication with a base station and to respond to paging channel requests.
The forward CDMA channel consists of a pilot channel, a sync channel, up to seven paging channels, and up to sixty-three forward traffic channels. Each of these channels is orthogonally spread by an appropriate Walsh function and then spread by the quadrature pair of PN sequences (I and Q) at a fixed rate of 1.2288 million chips per second.
The base station of a sector continuously transmits on the pilot channel of each active forward CDMA channel. A wireless unit operating within the coverage area of the base station uses this continuous transmission for synchronization. The network assigns each base station of the cell site a specific time (or phase) offset of the pilot PN sequence to identify a forward CDMA channel. A given base station uses the same pilot PN sequence offset, or simply PN offset, for all CDMA frequency assignments. For example, all traffic, sync, and paging channels transmitted from a single base station share the same PN offset. An offset index (0 through 511 inclusive) identifies distinct pilot channels. This offset index specifies the offset value from the zero offset pilot PN sequence. Each offset index increment represents the interval between pilot channels in increments of 64 chips (i.e., 52.08 ms).
An active wireless unit maintains four sets of pilot channels when communicating with a base station of a CDMA sector: the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. The Active Set contains all of the pilot channels that the wireless unit currently uses for demodulation. The Candidate Set contains all of the pilot channels that are not currently in the Active Set but have sufficient signal strength to be considered for soft or softer handoff. The Neighbor Set contains pilot channels that are not currently in the Active or Candidate Set, but may become eligible for handoff (e.g., pilot channels of nearby sites). The Remaining Set is the set of all possible assigned pilot channels in the CDMA network on the same carrier frequency, excluding the pilot channels defined in the other three sets.
Because all PN offsets in a network are time shifted versions of each other, it follows that with appropriate time delay, an incorrect pilot channel from any sector may be mistaken for a pilot channel in the Active Set. A large time delay between a wireless unit and a base station implies a large path loss, however, and hence a weak pilot channel signal at the wireless unit. Thus, if the PN offsets of different sectors have a large separation between them, a pilot channel signal would have a very high path loss and hence a very small probability of appearing within an active search window of another pilot channel. This makes an appropriate assignment of PN offsets crucial to ensure that a wrong pilot channel would be sufficiently weak to not cause any problems.
Reusing PN offsets is possible if: (1) a wireless unit being served by a base station is not interfered with by the pilot channel of another CDMA base station using the same PN offset, or (2) a base station in the network can uniquely identify all the pilot channel signals being reported by a wireless unit that it is serving.
The wireless unit uses a network-selected PILOT_INC parameter for the base station to determine which pilot channels to scan from among the Remaining Set. The Remaining Set includes the set of all possible pilot channels in the system that are integer multiples of the PILOT_INC parameter on the current CDMA frequency assignment, excluding pilot channels in the other sets. A Remaining Set pilot channel is assigned a lower priority in the scanning order, than an Active, Candidate or a Neighbor Set pilot channel.
The setting of the PILOT_INC parameter by the network is crucial to PN offset index planning. It impacts the wireless unit""s Remaining Set pilot channel scanning rate, the amount of co-offset and adjacent offset protection available in the network, and the total number of offsets available. Co-offset protection relates to interference caused by two or more sectors using the same PN offset. Adjacent offset protection relates to interference caused by two or more sectors using adjacent PN offsets.
The PILOT_INC parameter refers to the separation in phase or distance in phase-space between two adjacent PN offsets. It is an integer with a valid range from 1 to 15. Low values of PILOT_INC provide good co-offset protection, more offsets from which to choose, and less reuse of offsets, and increase the time to scan the pilot channels in the Remaining Set of the wireless unit. High values of PILOT_INC, on the other hand, provide good adjacent-offset protection, fewer offsets from which to choose, and more reuse of offsets, and decrease the time to scan the pilot channels in the Remaining Set of the wireless unit.
Conventional PN offset planning methods are based on idealized hexagonal grid structures, and accomplished by fitting a highly irregular pattern of cellular base station locations to a tessellated hexagonal grid pattern. PN offsets are assigned by reusing the same PN offset a specified predetermined number of base stations away.
In order to plan for growth, the conventional methods usually group PN offsets into a few groups, typically three for tri-sectored sites in the network. One sector of a base station is assigned a PN offset from one of the three groups. The other two sectors of the base station are assigned PN offsets from the other two groups, respectively. A few PN offsets in each group are reserved for growth of the network when new base stations are added. The remaining PN offsets in each group are used to make assignments. Making assignments by groups, however, is not optimal in terms of reducing inter-sector interference. Moreover, the unused PN offsets constitute a wasted resource until the network grows.
The conventional methods make initial assignment choices very easy because the assignments can be made without computer assistance or optimized planning. After an initial assignment, the methods account for irregularities through manual modification of the assignment by an experienced engineer with local knowledge of the environment. Unfortunately, PN offset planning under these idealized assumptions creates many inefficiencies due to the initial assignment""s inaccurate reflection of reality.
The irregularities that produce these inefficiencies are due to several factors. First, the need for base stations in a particular area is highly non-uniform because people do not tend to distribute themselves uniformly over large areas. People tend, for example, to cluster in neighborhoods, at work, and in cities. Second, choices for new base station locations are very limited due to factors such as zoning. Base station locations cannot be chosen in ideal locations even if the user traffic was uniformly distributed over a geographic area. Lastly, areas that are covered by base stations are highly dependent upon the propagation environment. Irregularities such as terrain, morphology, and reflecting structures produce highly irregular areas of coverage.
Automatic PN offset planning has heretofore found only limited application due to the need to account for several sets of constraints in a timely manner. Optimization systems must be able to handle large cellular networks with constraints on both co-offset and adjacent offset assignments. For example, the same PN offset cannot be used by neighbors of a base station or neighbors of neighbors of a base station. Adjacent offset protection must be provided to ensure that an adjacent offset does not propagate into a coverage area with significant power to interfere with the pilot channel. Both allowable PN offset separation and interference limits are constraints needed to address this problem adequately.
Many conventional systems address optimization problems using only minimum allowable PN offset separation constraints. These systems tend to be overly constraining since a range of PN offset prohibitions is needed between base station sectors rather than simply prohibiting use of every offset below a certain threshold interference. These systems also ignore factors such as interference from more than two assignments of the same offset.
By constraining the problem with minimum allowed offset separation between sectors, these systems tend to over constrain the PN offset assignment problem. For PN offset planning, two sectors interfere if the pilot channels arriving at the wireless unit are in phase with each other within the search window of the wireless unit. The systems look at all possible wireless unit locations to find a range of invalid PN offset separations that they use to constrain the problem. The constraint on PN offset separation, however, results in a range of allowable PN offsets rather than a minimum allowed PN offset separation.
Other conventional systems handle interference as well as separation constraints. These systems are designed, however, for the different problem of making analog frequency assignments. Such problems tend to be large and arise from the need to plan for many assignments per analog frequency. These systems must use less complex methods than those that can be used for PN offset planning.
None of the conventional systems or methods provides optimum PN offset assignment because none of these systems or methods considers all possible interference mechanisms in the network, constraints on co-offset and adjacent offset protection, CDMA border and beacon sites, and preassigned PN offsets. In addition, none of the conventional systems determine interference-prone and problematic areas within the serving area of a CDMA network. These problematic areas affect optimal PN planning in the network because these areas may lead to: synchronization of a wireless unit with the wrong pilot channel, pilot pollution, deterioration in voice and data signal quality and, thus, quality of service (QoS), and poor network performance.
Therefore, a need exists to provide information regarding interference-prone and problematic areas in the network so that overall network performance can be improved and optimum PN offset assignments can be made.
Systems and methods consistent with the present invention address this need by locating and addressing interference-prone and problematic areas in the network to facilitate optimum PN offset assignment. The systems and methods also enable scalability for future growth without the need for a revised network-wide retuning.
In accordance with the purpose of the invention as embodied and broadly described herein, a system, in one implementation consistent with the present invention, assigns PN offsets in a network with multiple sectors. The system sets parameters for the network and assigns PN offsets to the sectors based on the set parameters. The system then identifies sectors in the network that have poor network performance as a result of the PN offset assignments. The system changes parameters for the identified sectors and reassigns PN offsets to the sectors based on the changed parameters to improve network performance.
In another implementation consistent with the present invention, a method assigns PN offsets in a network with multiple sectors. The method includes determining potential interference between each pair of the sectors in the network; identifying constraints for each of the sectors based on the determined potential interference; assigning PN offsets to the sectors based on the identified constraints; assessing network performance as a result of the PN offset assignments; receiving a change to the PN offset assignment for at least one of the sectors; and reassessing network performance as a result of the received change.
In yet another implementation consistent with the present invention, a method assigns PN offsets in a network with multiple sectors. The method includes setting parameters for the network; assigning PN offsets to the sectors based on the set parameters; identifying at least a first sector in the network having poor network performance; identifying at least a second sector in the network causing interference to the first sector; changing parameters for at least one of the first and second sectors; and reassigning PN offsets to the sectors based on the changed parameters to improve network performance.